1. Field of the Invention
The invention relates to an electrochemical storage of the sodium and sulfur type with an anode space and a cathode space which are separated from each other by a solid electrolyte and are bounded at least in places, by a metallic housing, with the cathode space filled with fiber material of graphite or carbon which is saturated with sulfur for forming the sulfur electrode.
2. Description of the Prior Art
Such electrochemical storage cells are highly suited as energy sources. They find increasing use in the construction of storage batteries which are provided as a power supply of electric vehicles.
A specific example of these storage cells are those of the sodium and sulfur type which are rechargeable and have a solid electrolyte of beta aluminum oxide which separates the anode space from the cathode space. It should be pointed out as an advantage of these storage cells that no electrochemical secondary reactions proceed while they are being charged and the current yield is therefore near 100%. In such storage cells, the anode space contains sodium within the cup-shaped solid electrolyte. The cathode space is situated between the solid electrolyte and the metallic housing which defines the storage cells toward the outside. In the storage cells known to date, there is arranged within the cathode space a long-fiber material of graphite or carbon that is saturated with sulfur for forming the sulfur electrode. In the manufacture of the storage cells, elements in the form of half shells are formed from the fiber-shaped material, are impregnated with sulfur and then inserted into the cathode space. The storage cells are fabricated at room temperature. For operation, the storage cells are heated to a temperature of about 350.degree. C. A storage cell subjected to such a temperature influence causes the fiber material therein to expand, particularly that of the two half shells which are arranged in the cathode space. They expand to the extent that their end faces abut flush against each other and the fibers of the one half extend into the fibers of the other half, so that there is no longer any space in the boundary region of the half shells. When the storage cells are discharged, the sodium ions contained in the anode space migrate through the solid electrolyte into the cathode space and there form with the sulfur present the sodium polysulfide. Due to the fact that the two half shells formed of the long-fiber material now are in close contact with each other, the sodium polysulfide can distribute itself uniformly, including particularly over the boundary surfaces of the two half shells in the cathode space. If such a storage cell containing large amounts of sodium polysulfide in the cathode space is cooled down, the sodium polysulfide solidifies and forms a closed ring which firmly surrounds the solid electrolyte. Sodium polysulfide has a higher thermal coefficient of expansion than beta aluminum oxide, of which the solid electrolyte is made. This means that the ring of long-fiber material containing the sodium polysulfide shrinks onto the solid electrolyte when it cools down. Thereby, such long-fiber-sodium polysulfide impregnated ring adheres very strongly on the outside surfaces of the solid electrolyte and with declining temperature, exerts forces on the same, which is caused by the different thermal expansion coefficients of the two materials. These shearing forces can lead in due time to the formation of cracks in the solid electrolyte and thereby to the destruction of the entire storage cell.